material world without itself being visible.

To say that light is invisible, however, does not mean that it is
wholly imperceptible. It is difficult to bring the perception of light
into consciousness, for naturally our attention, when we look out into
light-filled space, is claimed by the objects of the illuminated world,
in all their manifold colours and forms. Nevertheless the effect of
pure light on our consciousness can be observed during a railway
journey, for instance, when we leave a tunnel that has been long enough
to bring about a complete adaptation of the eyes to the prevailing
darkness. Then, in the first moments of the lightening of the field of
vision, and before any separate objects catch the attention, we can
notice how the light itself exercises a distinctly expanding influence
on our consciousness. We feel how the light calls on the consciousness
to participate, as it were, in the world outside the body.

It is possible also to perceive directly the opposite of light. This is
easier than the direct perception of light, for in the dark one is not
distracted by the sight of surrounding objects. One need only pay
attention to the fact that, after a complete adapting of the eyes to
the dark, one still retains a distinct experience of the extension of
the field of vision of both eyes. We find here, just as in the case of
light, that our will is engaged within the eye in a definite way; a
systolic effect proceeds from dark, a diastolic effect from light. We
have a distinct perception of both, but not of anything 'visible' in
the ordinary sense.

With regard to our visual experience of white and black, it is quite
different. We are concerned here with definite conditions of corporeal
surfaces, just as with other colours, although the conditions conveying